This invention relates generally to satellite-based communication systems and methods and, in particular, relates to mobile satellite communication systems and methods.
Satellite Telephone Systems are rapidly emerging as a global business. These systems typically utilize many individual circuits that are routed through a constellation of satellites in low earth orbit (LEO) to effect communications. One important advantage of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth, without requiring the deployment of many small terrestrial cells.
After bands of frequencies were made available for the satellite telephone systems, one approach that was taken to provide multiple access capability used Time Division Multiple Access (TDMA), while a second approach used Code Division Multiple Access (CDMA). An attractive feature of the CDMA approach is an ability to share the frequencies by co-frequency operation, wherein a plurality of satellite repeaters operate on the same frequency. Combined with Low Earth Orbit (LEO) operation, and receiver designs having an ability to simultaneously receive multiple copies of the same data from multiple satellites, this feature allows a user terminal to receive signals for one, two, or more satellite repeaters simultaneously. This type of operation can be referred to as satellite diversity reception.
The constellation of satellites may be conveniently viewed as a large capacity of circuits in space, much like a distributed battery. That is to say, the sum of the stored power on the satellites (in terms of amp hours of real batteries), and the instantaneous power available from the deployed solar panels, yields the ability of the constellation of LEO satellites, or more generally the constellation of non-geosynchronous orbit (NGSO) satellites, to deliver communication circuit capacity to a region or succession of regions.
As can be appreciated, in order to successfully operate the satellite communication system for extended periods, with various satellites passing into and out of high traffic and low traffic areas at various times, it is important to correctly plan and allocate the usage of the various satellites to insure that all terrestrial service areas always have an adequate amount of coverage.
One method of selectively assigning user terminals to certain satellite repeaters to optimize, at least to a first order, the utilization of the satellite constellation is disclosed in U.S. Pat. No. 5,867,109, entitled xe2x80x9cSatellite Repeater Diversity Resource Management Systemxe2x80x9d, Robert A. Wiedeman. In this approach to solving the satellite allocation problem the system optimizes satellite path diversity to individual user terminals, and may deny or limit the use of diversity, based on one or more factors such as the location of user terminal and a radio propagation environment in the vicinity of the user terminal""s location.
Reference can also be made to U.S. Pat. No. 5,867,109, entitled xe2x80x9cMultiple Satellite Repeater Capacity Loading with Multiple Spread Spectrum Gateway Antennasxe2x80x9d, by Robert A. Wiedeman et al., which describes methods for assigning satellites to user terminals by considering elevation angle and other criteria.
A first object and advantage of this invention is to provide a system and a method for optimizing the capacity, reducing the satellite power, and improving the quality of the service delivered to users of a mobile communication satellite system.
It is a further object and advantage of this invention is to provide a mobile satellite communications system and method wherein available satellite repeaters are allocated and deallocated on the forward link transmission to user terminals based on a received signal quality indication, preferably a Frame Error Rate (FER) at the user terminal and then transmitted back to the ground station. Alternatively, FER may be determined at a ground station, based on return link quality and assuming that reverse link signal propagation conditions correlate with forward link signal propagation conditions.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
Disclosed herein is a a mobile satellite telecommunications system, and a method for operating the system, which will provide a real time method of accomplishing satellite constellation power optimization. The method comprises the steps of (a) providing a user terminal that is operable to simultaneously receive a communication signal from a ground station or gateway via a plurality of satellites; and, (b) at the ground station, selectively transmitting the communication to one satellite or a plurality of satellites each having a coverage area that includes a current location of the user terminal. In accordance with this invention the step of selectively transmitting includes a step of considering a plurality of factors, including a current reception state of the user terminal, a current state of satellite power of at least one of the plurality of satellites, and a predicted required state of satellite power of the at least one satellite.
The current reception state of the user terminal is indicated by a received signal quality indicator, which in the preferred embodiment is expressed as the Frame Error Rate (FER).
In one embodiment the FER is determined in the user terminal from a forward link transmission from the gateway, and the method further includes a step of sending an indication of the FER to the ground station using a reverse link transmission. In another embodiment the FER is determined in the ground station from a reverse link transmission from the user terminal. Preferably the indication of the Frame Error Rate is based on a relationship of a current value of the Frame Error Rate to previous values of the Frame Error Rate. If the value of the FER increases above a threshold level the number of selected satellites is increased, thereby increasing satellite diversity, and if the FER decreases below the threshold level the number of selected satellites is decreased, thereby decreasing satellite diversity.